LeetCode.206(反转链表) - 简单

LeetCode.206(反转链表) - 简单

• 反转一个单链表。

• 示例：

  输入: 1->2->3->4->5->NULL
  输出: 5->4->3->2->1->NULL
  

• 代码1:

  //2020_04_21
  //双指针
  //null   5 -> 4 -> 3 -> 2 -> 1 -> null
  //  cur  pre
  //null <- 5 -> 4 -> 3 -> 2 -> 1 -> null
  //       cur  pre
  //...
  //null <- 5 <- 4 <- 3 <- 2 <- 1    null
  //                           cur   pre
  /**
   * Definition for singly-linked list.
   * struct ListNode {
   *     int val;
   *     ListNode *next;
   *     ListNode(int x) : val(x), next(NULL) {}
   * };
   */
  class Solution {
  public:
      ListNode *reverseList(ListNode *head)
      {
          ListNode *cur = NULL;
          ListNode *pre = head;
          ListNode *t;
          while(pre){
              t = pre->next;
              pre->next = cur;
              cur = pre;
              pre = t;
          }
          return cur;
      }
  };
  

• 代码2：

  //2020_04_21
  //栈
  /**
   * Definition for singly-linked list.
   * struct ListNode {
   *     int val;
   *     ListNode *next;
   *     ListNode(int x) : val(x), next(NULL) {}
   * };
   */
  class Solution {
  public:
      ListNode* reverseList(ListNode* head) {
          if(!head) return NULL;//head为空
          ListNode* res, *t;
          stack  s;
          while(head){//依次压栈
              s.push(head -> val);
              head = head -> next;
          }
          res = new ListNode(s.top());//由于没有头节点，先取出第一个
          s.pop();
          t = res;
          while(!s.empty()){//依次取出
              t -> next = new ListNode(s.top());
              t = t -> next;
              s.pop();
          }        
          return res;//返回结果
      }
  };
  

• 结果：